Lithium secondary cells have characteristics of high energy density, long life and the like. Hence, the lithium secondary cells are used broadly as power supplies for household appliances such as video cameras, portable electronic devices such as laptop computers and cellular phones, and electric tools such as power tools, and recently, have also used as large-size cells mounted on electric vehicles (EVs), hybrid electric vehicles (HEVs) and the like.
Lithium secondary cells are secondary cells having a following structure. In the charge time, lithium dissolves out as ions from a positive electrode and migrates to a negative electrode and is intercalated therein. On the other hand, in the discharge time, lithium ions reversely return from the negative electrode to the positive electrode. Then, their high energy density is known to be due to potentials of their positive electrode materials.
As positive electrode active substances of this kind of lithium secondary cells, there are known, in addition to lithium transition metal oxides having a layer structure such as LiCoO2, LiNiO2 and LiMnO2, spinel-type lithium manganese-containing complex oxides having a manganese-based spinel structure (Fd-3m), such as LiMn2O4 and LiNi0.5Mn1.5O4.
The spinel-type lithium manganese-containing complex oxides, because of being low in raw material prices and being nontoxic and safe, and furthermore having a property durable to overcharge, are paid attention to as next-generation positive electrode active substances for large-size cells for electric vehicles (EVs), hybrid electric vehicles (HEVs) and the like. Further the spinel-type lithium transition metal oxides (LMOs) capable of three-dimensionally intercalating and deintercalating Li ions, because of being superior in output characteristics to the lithium transition metal oxides having a layer structure such as LiCoO2, are expected to be utilized for applications requiring excellent output characteristics, such as cells for EVs, cells for HEVs and the like.
In recent years, LiMn2O4 has been known to have an operating potential of nearly 5 V by substituting some of Mn sites therein with other transition metals (Cr, Co, Ni, Fe, Cu), and there are at present actively carried out developments of (5 V-class) manganese-based spinel-type lithium transition metal oxides having an operating potential of 4.5 V or higher.
For example, Japanese Patent Laid-Open No. 11-73962 discloses, as a positive electrode active substance of a lithium secondary cell exhibiting an electromotive force of 5 V-class, a high-capacity spinel-type lithium manganese complex oxide positive electrode active substance made by adding chromium as an essential component to a spinel-type lithium manganese complex oxide, and further adding nickel or cobalt.
Japanese Patent Laid-open No. 2000-235857 discloses a crystal LiMn2−y−zNiyMzO4 (wherein M is at least one selected from the group consisting of Fe, Co, Ti, V, Mg, Zn, Ga, Nb, Mo and Cu; and 0.25≦y≦0.6 and 0≦z≦0.1) having a spinel structure, which carries out charge and discharge at a potential of 4.5 V or higher vs. Li metal.
Japanese Patent Laid-Open No. 2003-197194 discloses, as a positive electrode material for a high-energy density lithium-ion secondary cell having a high voltage of 4.5 V or higher vs. Li, a spinel-type lithium manganese complex oxide represented by Lia(MxMn2−x−yAy)O4 (wherein 0.4<x, 0<y, x+y<2 and 0<a<1.2; M contains one or more metal elements containing at least Ni selected from the group consisting of Ni, Co, Fe, Cr and Cu; and A contains at least one metal element selected from Si and Ti, and in the case where A contains Ti alone, the ratio y of A is 0.1<y).
Japanese Patent Laid-Open No. 2004-22095 discloses a positive electrode active substance for a nonaqueous electrolyte secondary cell represented by the general formula: LizCo1−x−yMgxMyO2 wherein the element M is at least one selected from the group consisting of Al, Ti, Sr, Mn, Ni and Ca; and x, y and z satisfy (i) 0≦z≦1.03, (ii) 0.005≦x≦0.1 and (iii) 0.001≦y≦0.03, wherein the oxide has a crystal structure assigned to a hexagonal system in an overcharge region having a potential of higher than 4.25 V vs. metallic Li; and in a gas chromatography/mass spectroscopy of the oxide in the overcharge region, the maximum value of the oxygen evolution peak is in the range of 330 to 370° C.
A spinel-type lithium cobalt manganese complex oxide (referred to as “Co-based 5 V-class spinel”) made by substituting some of Mn sites in LiMn2O4 with metal elements mainly containing Co to be thereby enabled to have an operating potential of 4.5 V or higher is characterized by having a high potential. It is difficult, however, to simultaneously raise its capacity, and then a problem of the oxide is that it is not easy to enhance the energy density.
Then the present invention relates to a spinel-type lithium cobalt manganese complex oxide made by substituting some of Mn sites in LiMn2O4 with metal elements mainly containing Co, and is to provide a novel spinel-type lithium manganese-containing complex oxide not only having an operating potential of 4.5 V or higher at a metal Li reference potential but also being capable of extending its capacity region of 5.0 V or higher and being capable of enhancing its energy density as well.